Ultra-wideband (UWB) communications systems are normally defined as carrier-less communications systems wherein the bandwidth of the signal being transmitted, fB, is greater than or equal to 0.20 fc, where fc is the center frequency of the signal being transmitted and has a minimum bandwidth of 500 MHz. Note that this definition is specified by the Federal Communications Commission of the United States. Narrowband communications systems will have a signal bandwidth to center frequency ratio significantly less than that. For example, IEEE 802.11b, a popular narrowband communications system that transmits in the 2.4 GHz ISM (Industrial, Scientific, and Medical) band, has a signal bandwidth of less than 25 MHz. In other words, IEEE 802.11b's signal bandwidth, fB, is on the order of 0.01 fc.
UWB communications systems have been around for a great number of years, and the majority of them fall under one particular classification, they modulate a stream of short-duration pulses (with an approximate duration of 0.2 nanoseconds (ns)), either in time (pulse position modulation (PPM)), amplitude (pulse amplitude modulation (PAM)), or phase angle (bi-phase modulation). While the existing UWB communications systems can achieve reasonably good performance under ideal conditions, the systems also have significant disadvantages.
The Federal Communications Commission (FCC) of the United States has recently approved the use of UWB communications systems in the US in a report and order entitled “In the matter of Revision of Part 15 of the Commission's Rules, Regarding Ultra-Wideband Transmission System, ET Docket 98-153”, adopted Feb. 14, 2002, released Apr. 22, 2002. The document is herein incorporated by reference. The report and order requires that UWB systems used in communications systems operate in the 3.1 to 10.6 GHz frequency band and are limited to indoor use or in hand-held devices that can be employed in activities such as peer-to-peer activities. The FCC also specifies a maximum transmit power for the UWB communications systems.
Since the FCC did not restrict UWB communications systems to the use of short duration pulse streams, a variety of UWB communications systems have been proposed. By necessity, these UWB communications systems meet the FCC restrictions. However, they do not all use short duration pulse streams. Rather, they use techniques such as orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) and other methods to help create a high data rate communications network.
Several UWB communications systems combine both OFDM and CDMA together to feature advantages of both techniques in a single communications system. For example, in a typical UWB communications system, data to be transmitted would be spread via multiplication with spreading codes (CDMA) followed by compensation, padding, and conversion into a time domain signal and placed into small frequency ranges (OFDM) prior to transmission. The combination of CDMA and OFDM can increase bandwidth utilization and increased interference resistance while minimizing impact upon adjacent communications networks. For example, OFDM can permit maximization of the use of available bandwidth at different frequency ranges throughout the transmission band by placing as much of the data to be transmitted into each of the multiple small frequency ranges, while the use of CDMA can help to minimize the impact of the transmissions on other communications systems which may be operating within the general vicinity. At the receiver, the CDMA and OFDM would be removed to obtain the transmitted data.
A disadvantage of the prior art is the use of CDMA followed by OFDM in a communications network can result in the need to perform a Fourier transform at chip rate, which can be significantly higher than the actual data rate. Since a Fourier transform can require a large amount of computational power (multiplication and addition of complex numbers), computing a Fourier transform at a rate that is higher than necessary can result in the need for greater computational power. Hence, overall costs (due to the more powerful processing unit) can be higher than necessary.
A second disadvantage of the prior art is that the greater computational power needed by performing the Fourier transform at a rate that is higher than necessary is that the greater power processing unit, combined with the higher clock rate, will typically consume more power. Therefore, the overall power consumption will be greater. In a wireless receiver, this may necessitate a larger battery or a more advanced (and more expensive) battery technology.